So that each type of text file can be treated appropriately, emacs
supports various major modes of operation.
In the status line you will see something like

--:** emacs.tex (TeX)--L1-----16%---------

The major mode of the buffer is in brackets. emacs tries to pick the
most appropriate mode by looking at the file name, but you can explicitly set
modes. For example, you can type M-x C-mode to enter a mode with
support for those writing C programs.

Minor modes are mutually independant and independant of major modes. For
instance, if you type M-x auto-fill-mode, the status line will display
something like

--:** emacs.tex (TeX Fill)--L1-26%-------

and emacs will create a new line for you when you reach the right edge
of your window.

To find out which modes are available and what they provide,
look in emacs' Info files.